How to do exactly same thing as array_to_json(array_agg(tags.*)) for N columns

Question

I am currently using PostgreSQL JSON capabilities to create JSON objects out of my query so I can easily use it on my application or pass it to the frontend.

array_to_json(array_agg(tags.*)) does exactly when I need to (creates JSON objects with columns as a keys from the data and convert it into array), however I haven't found any way how to do the same if I need only one or two columns from tags. I played with various JSON and array functions but I've never achieved the same result. Thanks for help

Whole query

SELECT
    tags_components.component_id,
    array_to_json(array_agg(tags.*)) as tags
FROM tags_components 
LEFT JOIN tags ON tags.id = tags_components.tag_id 
AND tags_components.component_name = 'company'
GROUP BY tags_components.component_id

Answer 1

Use a derived table, e.g.:

SELECT
    tags_components.component_id,
    array_to_json(array_agg(tags.*)) as tags
FROM tags_components 
LEFT JOIN (
    SELECT id, name -- only two columns
    FROM tags 
    ) tags
ON tags.id = tags_components.tag_id 
AND tags_components.component_name = 'company'
GROUP BY tags_components.component_id